Howdy.
Is there any official ruling or barrier to upgrading the various weapon "modes" of a Rod of Lordly Might?
For example...
When button 1 is pushed, the rod becomes a +1 flaming longsword
Can that be enchanted further to be a +3 keen flaming burst longsword?
The only rule allowing it is rule 0, the GM/table come up with how they want it to work. The official rule is that you can't.
I think the default is that you can upgrade or combine items, although your GM has final say on both the cost or whether it is allowed at all.
Upgrading the weapon aspect on a rod of lordly might is unlikely to be straightforward - I haven't looked at the maths, but there may well be an increase in cost beyond simply the difference in price between the +1 flaming and +3 flaming burst.
Multiple Different Abilities: Abilities such as an attack roll bonus or saving throw bonus and a spell-like function are not similar, and their values are simply added together to determine the cost. For items that take up a space on a character's body, each additional power not only has no discount but instead has a 50% increase in price.
This suggests that the sword enchantment is simply added on. The ROLM doesn't take up a "slot", but is a handheld item, and so the 50% increase isn't applicable.